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    Bindings bug or feature

    QML and Qt Quick
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    • Q
      QtYury last edited by

      I found a situation, where bindings works not as I expected. If I bind variable someVar to some other variable(s) and also add handler onSomeVarChanged, the handler executes before bindings. Thus, using any non-trivial code in handler onSomeVarChanged is potentially dangerious. You can use some component, that has variable binded to someVar and this variable is not updated yet. See example below:

      main.qml
      @
      import QtQuick 2.3
      import QtQuick.Controls 1.2
      import QtQuick.Layouts 1.1

      ApplicationWindow {
      id: window

      property var currentDate: null
      
      width: 640
      height: 480
      
      onCurrentDateChanged: {
          print("window.currentDate = " + currentDate)
          print("requestManager.date = " + requestManager.date)
          requestManager.requestData()
      }
      
      RequestManager {
          id: requestManager
          date: currentDate
      }
      
      Button {
          text: "Click Me!"
          onClicked: currentDate = new Date
      }
      

      }
      @

      RequestManager.qml
      @
      import QtQuick 2.0

      Item {

      property var date;
      
      function requestData() {
          print("requesting data for date " + date)
      }
      

      }
      @

      Output (after button click):
      @
      qml: window.currentDate = Пт сен 19 13:26:24 2014 GMT+0300
      qml: requestManager.date = null
      qml: requesting data for date null
      @

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      • Q
        QtYury last edited by

        Of course, there are a lot of workarounds. For example:

        main.qml
        @
        import QtQuick 2.3
        import QtQuick.Controls 1.2
        import QtQuick.Layouts 1.1

        ApplicationWindow {
        id: window

        property var currentDate: null
        
        width: 640
        height: 480
        
        function setCurrentDate(newDate) {
            currentDate = newDate
            requestManager.requestData()
        }
        
        RequestManager {
            id: requestManager
            date: currentDate
        }
        
        Button {
            text: "Click Me!"
            onClicked: setCurrentDate(new Date)
        }
        

        }
        @

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        • V
          Vincent007 last edited by

          how about this? Is it better?

          @
          ApplicationWindow {
          id: window

          property var currentDate: null
          
          width: 640
          height: 480
          
          RequestManager {
              id: requestManager
              date: currentDate
              onDateChanged: {
                  print("window.currentDate = " + currentDate)
                  print("requestManager.date = " + date)
                  requestData()
              }
          }
          
          Button {
              text: "Click Me!"
              onClicked: currentDate = new Date
          }
          

          }
          @

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          • Q
            QtYury last edited by

            Thank you, Vincent. Maybe, it is better in some way.
            But what if we need to execute some code after requestManager.requestData(), for example:

            @
            onCurrentDateChanged: {
            requestManager.requestData()
            viewManager.updateViews()
            // ... etc ...
            }
            @

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            • C
              chrisadams last edited by

              Hi,

              One important feature of a declarative language is that you don't know in which order the declarations will be applied in, or the order the side-effects (signal handlers, binding expressions) will be evaluated in. In practice, there is an order defined by the engine implementation, but it is best not to rely on that, as it is an implementation detail rather than a well-specified behaviour (ie, API contract).

              To ensure that an imperative expression is evaluated after a binding evaluation occurs, you can trigger the imperative function via a Timer started as a side-effect of the binding expression.

              Example:

              @
              property bool triggerUpdate: false
              property var someProperty: {
              var newValue = Math.random();
              if (triggerUpdate) {
              timer.start();
              }
              return newValue;
              }
              Timer {
              id: timer
              interval: 0 // will trigger upon returning to the Qt Event Loop
              onTriggered: {
              updateViews() // or whatever
              }
              }
              Component.onCompleted: triggerUpdate = true; // will trigger re-evaluation of someProperty binding, and anything bound to someProperty
              @

              In the preceding case, the updateViews() function is guaranteed to be called after ALL binding expressions involving someProperty have been re-evaluated, since the Timer's onTriggered handler will be invoked at some point in the future via the Qt Event Loop.

              I hope this helps!

              Cheers,
              Chris Adams.

              http://www.qinetic.com.au - The Qt And QML Experts

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